There are two methods generally used as architecture for wireless receivers, a direct-conversion method and a super-heterodyne method. In the direct-conversion method, frequency of a signal carrier is dropped to direct current level by one step. In the super-heterodyne method, a few units at intermediate frequencies are provided to drop the frequency gradually.
The super-heterodyne method has an image signal problem. More specifically, when an input signal is dropped to a signal having an intermediate frequency, a signal (image signal) having a frequency opposite to a desired signal may overlap with the desired signal. If the image signal overlapping with the desired signal is demodulated, signal noise ratio (SNR) may be decreased.
To avoid such a problem, a filter is generally provided in steps before a frequency conversion step by a mixer to perform a predetermined filtering process. As a result, the image signal is fully suppressed and input to the mixer so that the image signal does not overlap with the desired signal.
There is increasing demand for a one-chip solution for recent receivers using Complementary Metal Oxide Semiconductor (CMOS). However, it is difficult to achieve a high-performance filter with the CMOS technology. Further, the one-chip solution requires larger chip size.
Recently, a technology generally called “Low-IF” has been proposed. In the Low-IF technology, the intermediate frequency is set to the lower frequency band. Using this Low-IF technology, a filter circuit and an Automatic Gain Control (AGC) circuit can be achieved easily with simple circuits because the circuits deal with a low intermediate frequency band. Thus, the Low-IF technology provides advantages in power consumption and cost.
If the intermediate frequency band is set below the frequency band used in the Analog-Digital (A/D) converter, the intermediate frequency can be captured into digital without conversion. Accordingly, following demodulation processing can be performed easily by digital circuits.
However, the image signal problem described previously becomes more significant when the Low-IF technology is used. More precisely, when the intermediate frequency is low enough to the carrier frequency, a filter is needed with sharp filtering characteristics at the carrier band. Accordingly, a high-performance filter must be prepared outside of the chip. Consequently, the number of parts increases, resulting in a cost penalty.
For the above-described problem, even if the image signal is included in the signal down-converted by the mixer, a technology using a poli-fuse filter is known to remove such image signal. In this technology, the image signal is removed by filtering with an I signal and a Q signal generated by down-converting the received signal using signals which have a phase difference of 90 degrees.
JP-2006-121665 describes using a poli-fuse filter to remove the image signal. The technology described in JP-2006-121665 achieves high-precision filtering characteristics at low power and low cost by providing a variable frequency filter capable of varying frequency at a following stage after the poli-fuse filter that removes the image signal.
However, the poli-fuse filter is formed of passive elements such as resisters and capacitors. Accordingly, when the intermediate frequency band is low, the passive elements require a large area. Further, receiving sensitivity may be decreased by noise caused by the poli-fuse filter itself.
JP-2546331 describes an active-type filter using elements such as transistors, resisters, and capacitors. However, such an active-type filter still has the same problem of increase in chip size because the time constant of the filter is still determined by the resister and the capacitor.
The Weaver method is well known as a technology to remove the image signal without using a poli-fuse filter. In the Weaver method, the I and Q signals are generated by down-converting the received signal using signals which have a phase difference of 90 degrees (i.e., are orthogonal to each other), and are added to remove the image signal.
When the Weaver method is used, the I and Q signals are required to be perfectly orthogonal and have equal gain. If there are some errors in the phase or the gain between the I and Q signals, it becomes imperfect to remove the image signal. As a result, receiving SNR may be decreased. This factor is important because, in actual CMOS devices, there is variation in operational characteristics of the transistors even if the transistors are formed with the same layout pattern. Thus, it is substantially impossible to match the I and Q signals perfectly without error. Accordingly, it is required to detect an error caused by the circuit and correct it.
To perform this correction operation, an image signal having a different frequency from a local oscillation signal necessary for down-converting by the mixer is required. Accordingly, an oscillator which is not used in the normal receiving operation is required, resulting in increase of chip size.
JP-2003-224002 describes a radio that receives a FM broadcast and an AM broadcast, with the intention of reducing the number of external parts in addition to reducing cost. A frequency of a crystal and an oscillation frequency of an oscillator generated using the crystal are determined so that the oscillation frequency becomes the least common multiple or an integral multiple of a reference frequency of a FM frequency synthesizer, an AM frequency synthesizer, a FM stereo demodulation circuit, and an AM synchronous detection circuit.
Requirement for phase and noise performance of the local oscillator for the AM broadcast is not so high compared to that for the FM broadcast because the AM frequency is lower than the FM frequency. Accordingly, it gives excessive performance to use the local oscillator of the FM broadcast for the AM broadcast, resulting in increase of power consumption.